This proposal aims to achieve a mechanistic understanding of p53 and ATM suppression in thymic lymphoma. The specific aims are: 1. Determine the mechanism by which p53 suppresses thymic lymphoma. The applicant will examine the checkpoints in stimulated p53+/+ primary thymocytes at various stages of differentiation. Tumors from p53-deficient mice will be examined for mutations in other known checkpoints. The consequences of inactivating a known mitotic checkpoint regulator, Bud 1, in vivo will be examined. 2. Determine the role of ATM in suppressing V(D)J-driven thymic lymphoma. The frequencies of interlocus recombination in ATM+/+ and +/- thymocytes and in B-cell precursors will be determined. She has already shown that interchromosoaml recombination is 10-100 fold higher than normal in ATM-/- thymocytes. She will determine whether the mechanism of ATM suppression is direct or indirect during V(D)J recombination. The impact of ATM-deficiency on cell cycle regulation of V(D)J recombination will be assessed. 3. Examine cooperation of ATM and p53 deficiencies in the development of thymic lymphoma. Tumors arising in doubly deficient mice will be analyzed for chromosomal abnormalities. Mice deficient in one gene and heterozygous for the other will be analyzed for loss of heterozygosity. Cell cycle analyses of doubly deficient thymocytes will be performed. 4. Identify oncogenic targets in ATM-deficient and p53-deficient lymphoma. Neither ATM nor p53 is sufficient to induce lymphoma; additional oncogenic events, presumably induced by translocation (ATM deficiency) or aneuploidy (p53 deficiency), are required. To determine the genes involved, differential expression analysis will be performed followed by functional analysis of candidate genes.